Steel powder composition and sintered body thereof

ABSTRACT

A powder composition and a sintered body thereof are presented. The powder is a martensitic stainless steel powder for powder injection molding without deformation problems during sintering. The powder composition includes 0.80-1.40 weight percent (wt %) of carbon (C), less than 1.0 wt % of silicon (Si), less than 1.0 wt % of manganese (Mn), 15.0-18.0 wt % of chromium (Cr), 0.10-2.50 wt % of titanium (Ti), and the remainder iron (Fe). The powder can be sintered with a sintering temperature varying within 50° C. and can reach a high density without distortion, and thereby a good dimensional stability is obtained.

FIELD OF INVENTION

The present invention relates to a steel powder composition and a sintered body thereof, and more particularly to a martensitic stainless steel powder composition for metal injection molding, which has an improved dimensional control, and a sintered body thereof.

BACKGROUND OF THE INVENTION

Metal injection molding (MIM) includes processes such as mixing a metal powder and a polymer binder, molding with an injection molding machine, debinding, and sintering at high temperatures, to obtain a metal part of near net shape. The technology involves in two fields, that is, powder metallurgy and plastic injection molding. Due to the high strength and hardness requirements for a MIM material, martensitic stainless steel is widely used, for example, steel species such as Japanese SUS410 series, Japanese SUS420 series, and Japanese SUS440C series.

However, the martensitic stainless steel powders generally have problems of poor sinterability, such as poor dimensional stability, the non-uniform sintered density, inconsistent properties from batches to batches, melting at the surface of sintered work pieces, and even distortion. The reason lies in that the optimum sintering temperature of the steel species falls within about 10° C. When the temperature is higher than this temperature range, the amount in liquid phase is excessive, and thus a network of liquid phase is formed, and the strength is lowered, and even distortion occurs. When the sintering temperature is lower than this temperature range, the sintered density is too low. At present, one of the solutions for solving these problems in sintering of the martensitic stainless steel powder is to control the temperature homogeneity of a sintering furnace within ±5° C. of the optimum sintering temperature, that is, the sintering window is 10° C. However, in that case, a several sets of thermocouples, heaters, and programmed controllers need to be mounted on the sintering furnace, and thus the cost is increased. If a small sintering furnace is used, good temperature uniformity may be achieved. However, the production rate is low.

SUMMARY OF THE INVENTION

In view of the problems above, the present invention is a steel powder composition and a sintered body thereof, to overcome the disadvantages of a conventionally sintered martensitic stainless steel powder such as poor mechanical properties, low sintered density, unstable dimension, and difficult temperature control. In order to achieve the objectives above, the steel powder composition of the present invention comprises 0.80-1.40 wt % of carbon, less than 1.0 wt % of silicon, less than 1.0 wt % of manganese, 15.0-18.0 wt % of chromium, 0.10-2.50 wt % of titanium, and the rest of iron.

The sintered body of the present invention is prepared from the steel powder composition of the present invention through a sintering process.

The steel powder composition of the present invention may further comprise 0.20-1.50 wt % of at least one of molybdenum, vanadium, and tungsten.

The sintered body of the present invention may further comprise 0.20-1.50 wt % of at least one of molybdenum, vanadium, and tungsten.

Titanium in the steel powder composition of the present invention may be originated from a pre-alloyed powder, a titanium powder, or a titanium-containing carbide powder.

The effect of the present invention lies in that, a titanium-containing composite carbide such as titanium carbide (TiC) or titanium-vanadium carbide ((Ti,V)C) is formed in the process by adding a strong carbide formation element, such as titanium, or by adding a titanium carbide such as TiC or (Ti,V)C, in the steel powder composition, to overcome the disadvantages such as poor dimensional control and low sintered density generated in sintering of the conventional martensitic stainless steel powder.

Furthermore, the steel powder composition of the present invention may improve the sintering temperature range to 50° C. and still achieve a high sintered density with good shape retention capability, and thereby the production yield is improved.

BRIEF DESCRIPTION OF THE DRAWINGS

The present invention will become more fully understood from the detailed description given herein below for illustration only, and thus are not limitative of the present invention, and wherein:

FIG. 1 shows a sintering characteristic of Comparative Embodiment 1;

FIG. 2 shows a sintering characteristic of Comparative Embodiment 2;

FIG. 3 shows a sintering characteristic of Comparative Embodiment 3;

FIG. 4 shows a sintering characteristic of Comparative Embodiment 4;

FIG. 5 shows a sintering characteristic of Embodiment 1 of the present invention;

FIG. 6 shows a sintering characteristic of Embodiment 2 of the present invention;

FIG. 7 shows a sintering characteristic of Embodiment 3 of the present invention;

FIG. 8 shows a sintering characteristic of Embodiment 4 of the present invention;

FIG. 9 shows a sintering characteristic of Embodiment 5 of the present invention; and

FIG. 10 shows a sintering characteristic of Embodiment 6 of the present invention.

DETAILED DESCRIPTION OF THE INVENTION

The implementation of the present invention is described in detail below. Table 1 shows the chemical compositions of the embodiments and comparative embodiments of the present invention. Embodiments 1 to 6 are the chemical compositions of the steel powder compositions of the present invention and the sintered body thereof, and Comparative Embodiments 1 and 2 are the chemical compositions of SUS440C martensitic stainless steels currently used in the industry and prepared by water atomization and gas atomization. Table 2 shows the temperature range of sintering windows of the comparative embodiments and embodiments of the present invention.

The sintering tests are performed as follows.

Comparative Embodiment 1

the alloy composition of this comparative embodiment as shown in Table 1 is of a commercially available SUS440C martensitic stainless steel pre-alloyed powder prepared by water atomization. The metal powder of Comparative Embodiment 1 is mixed with a suitable amount of graphite powder, such that a carbon content required by SUS440C is achieved after sintering. Then, the pre-mixed metal powder is further mixed with 7 wt % of a binder, mixed for 1 h in a Z-type high-shear-rate mixer at 150° C., and then cooled to room temperature to obtain a granular injection molding feedstock. Such a feedstock is charged in an injection molding machine and fabricated into a cylindrical test specimen having a diameter of 12.5 mm and a length of 20 mm. The injection molded test piece is debinded by a conventional debinding step in the industry to remove the binder and then sintered in a vacuum sintering furnace, where the temperature is raised from room temperature to 650° C. at a rate of 5° C./min and maintained at 650° C. for 1 h, and then the temperature is raised at a rate of 10° C./min to a pre-set sintering temperature and maintained for 1 h, and followed by cooling to 800° C., and then rapidly cooled with a fan.

The thermal homogeneity at the sintering temperature of the special sintering furnace used in the present invention may be controlled within ±5° C., and thus the total temperature range in the present invention is 10° C. The definition for the sintering window includes: a lower temperature limit at which a density of 98% or more of the theoretical density (which is about 7.72 g/cm³ for the SUS440C martensitic stainless steel) is achieved, and an upper temperature limit at which deformation of the sinter part occurs or the measured dimensions has a difference of 1% or above between the diameters at two ends of the sinter body.

FIG. 1 shows the sintering characteristic of Comparative Embodiment 1. The sintering window of Comparative Embodiment 1 is within 10° C., that is, within ±5° C.; however, such a sintering window is not suitable for a sintering furnace currently used in the industry (the thermal homogeneity of a common sintering furnace in the industry is about ±10° C.), because the production yield is low.

Comparative Embodiment 2

the alloy composition of this comparative embodiment is shown in Table 1. In this embodiment, a commercially available SUS440C martensitic stainless steel pre-alloyed powder prepared by gas atomization is subjected to the process of Comparative Embodiment 1, and FIG. 2 shows the sintering characteristics. The sintering window of Comparative Embodiment 2 is also within 10° C., that is, within ±5° C., and thus this comparative embodiment is still not suitable for being sintered in an industrial sintering furnace for large-scale production.

Comparative Embodiment 3

the alloy composition of this comparative embodiment is shown in Table 1, in which tungsten (W) is provided by adding 2.0 wt % of the tungsten carbide (WC) powder, and FIG. 3 shows the sintering characteristics. The sintering window of Comparative Embodiment 3 is also within 10° C., that is, within ±5° C., and thus this comparative embodiment is still not suitable for being sintered in an industrial sintering furnace for large-scale production.

Comparative Embodiment 4

the alloy composition of this comparative embodiment is shown in Table 1, in which chromium (Cr) is provided by adding 2.0 wt % of the chromium carbide (Cr₃C₂) powder, and FIG. 4 shows the sintering characteristics. The sintering window of Comparative Embodiment 4 is also within 10° C., that is, within ±5° C., and thus this comparative embodiment is also not suitable for being sintered in an industrial sintering furnace for large-scale production. The test piece of Comparative Embodiment 4 is likely to deform, indicating that the addition of Cr₃C₂ cannot improve the sintering behavior.

Embodiment 1

the alloy composition of this embodiment is shown in Table 1. In this embodiment, a titanium-containing pre-alloyed powder prepared by gas atomization is subjected to the process in Comparative Embodiment 1. FIG. 5 shows the sintering characteristics of this embodiment. The sintering window of Embodiment 1 is increased to 50° C., that is, within ±25° C. Such a sintering window improves the sinterability significantly, and is capable of being used in a common sintering furnace in the industry.

Embodiment 2

the alloy composition of this embodiment is shown in Table 1. In this embodiment, titanium (Ti) is provided by adding 1.0 wt % of the titanium carbide (TiC) powder, and this powder mixture is subjected to the process in Comparative Embodiment 1. FIG. 6 shows the sintering characteristics. The sintering window of this embodiment is increased to 20° C., that is, within ±10° C.

Embodiment 3

the alloy composition of this embodiment is shown in Table 1. In this embodiment, titanium (Ti) is provided by adding 2.0 wt % of the titanium carbide (TiC) powder, and this powder mixture is subjected to the process in Comparative Embodiment 1. FIG. 7 shows the sintering characteristics. The sintering window of this embodiment is increased to 40° C., that is, within ±20° C., indicating that the addition of titanium carbide (TiC) powder improves the sintering behavior.

Embodiment 4

the alloy composition of this embodiment is shown in Table 1. In this embodiment, titanium (Ti) and tungsten (W) are provided by adding 2.0 wt % of a titanium-tungsten composite carbide, (W,Ti)C, in which the weight ratio of WC/TiC is 50/50, and the powder mixture is subjected to the process in Comparative Embodiment 1. FIG. 8 shows the sintering characteristics. The sintering window of this embodiment is 20° C., that is, within ±10° C., and the sinterability of Comparative Embodiment 1 is improved.

Embodiment 5

the alloy composition of this embodiment is shown in Table 1. In this embodiment, titanium (Ti) is provided by adding 2.0 wt % of the titanium carbide (TiC) powder, and the powder mixture is subjected to the process in Comparative Embodiment 1. FIG. 9 shows the sintering characteristics. The sintering window of this embodiment is increased to 40° C., that is, within ±20° C.

Embodiment 6

the alloy composition of this embodiment is shown in Table 1. In this embodiment, titanium (Ti) is provided by adding a titanium-containing steel powder, and the powder mixture is subjected to the process in Comparative Embodiment 1. FIG. 10 shows the sintering characteristics. The sintering window of this embodiment is increased to 30° C., that is, within ±15° C., indicating that the addition of a titanium-containing pre-alloyed powder improves the sintering behavior.

TABLE 1 Steel wt. % Species C Si Mn Cr Mo V W Ti Fe Embodiment 1 1.18 0.68 0.85 17.10 0.10 — — 0.72 The balance Embodiment 2 1.05 0.81 0.76 16.99 0.04 — — 0.80 The balance Embodiment 3 1.22 0.81 0.76 16.99 0.04 — — 1.60 The balance Embodiment 4 1.08 0.81 0.76 16.99 0.04 — 0.93 0.80 The balance Embodiment 5 1.25 0.53 0.60 17.20 0.56 — — 1.60 The balance Embodiment 6 1.22 0.65 0.69 15.64 0.35 0.13 — 0.20 The balance Comparative 1.03 0.81 0.76 16.99 0.04 — — — The balance Embodiment 1 Comparative 0.98 0.53 0.60 17.20 0.56 — — — The balance Embodiment 2 Comparative 1.02 0.81 0.76 16.99 0.04 — 1.90 — The balance Embodiment 3 Comparative 1.05 0.81 0.76 18.73 0.04 — — — The balance Embodiment 4

TABLE 2 Embodiment 1 Embodiment 2 Embodiment 3 Embodiment 4 Sintering Sintered Sintering Sintered Sintering Sintered Sintering Sintered temperature density temperature density temperature density temperature density (° C.) (g/cm³) Shape (° C.) (g/cm³) Shape (° C.) (g/cm³) Shape (° C.) (g/cm³) Shape 1270 6.21 ◯ 1260 6.68 ◯ 1260 6.34 ◯ 1260 6.72 ◯ 1280 7.74 ◯ 1270 7.09 ◯ 1270 6.89 ◯ 1270 7.45 ◯ 1290 7.72 ◯ 1280 7.66 ◯ 1280 7.67 ◯ 1280 7.66 ◯ 1300 7.72 ◯ 1290 7.64 ◯ 1290 7.66 ◯ 1290 767.   ◯ 1310 7.74 ◯ 1300 7.61 X 1300 7.66 ◯ 1300 7.69 X 1320 7.73 ◯ 1310 7.65 ◯ 1330 7.69 X 1320 7.62 X Carbon 1.18 Carbon 1.05 Carbon 1.22 Carbon 1.08 content content content content Sintering Within 50° C. Sintering Within 20° C. Sintering Within 40° C. Sintering Within 20° C. window (1275-1325° C.) window (1275-1295° C.) window (1275-1315° C.) window (1275-1295° C.) Embodiment 5 Embodiment 6 Comparative Embodiment 1 Comparative Embodiment 2 Sintering Sintered Sintering Sintered Sintering Sintered Sintering Sintered temperature density temperature density temperature density temperature density (° C.) (g/cm³) Shape (° C.) (g/cm³) Shape (° C.) (g/cm³) Shape (° C.) (g/cm³) Shape 1260 6.48 ◯ 1250 7.10 ◯ 1260 6.85 ◯ 1250 6.59 ◯ 1270 7.05 ◯ 1260 7.34 ◯ 1270 7.21 ◯ 1260 7.12 ◯ 1280 7.71 ◯ 1270 7.71 ◯ 1280 7.69 ◯ 1270 7.52 ◯ 1290 7.71 ◯ 1280 7.69 ◯ 1290 7.71 X 1280 7.71 ◯ 1300 7.73 ◯ 1290 7.72 ◯ 1290 7.70 X 1310 7.72 ◯ 1300 7.70 X 1320 7.66 X Carbon 1.25 Carbon 1.22 Carbon 1.03 Carbon 0.98 content content content content Sintering Within 40° C. Sintering Within 30° C. Sintering Within 10° C. Sintering Within 10° C. window (1275-1315° C.) window (1265-1295° C.) window (1275-1285° C.) window (1275-1285° C.) Comparative Embodiment 3 Comparative Embodiment 4 Sintering Sintered Sintering Sintered temperature density temperature density (° C.) (g/cm³) Shape (° C.) (g/cm³) Shape 1230 7.02 ◯ 1260 6.71 ◯ 1240 7.53 ◯ 1270 7.12 ◯ 1250 7.78 ◯ 1280 7.72 ◯ 1260 7.77 X 1290 7.73 X Carbon 1.02 Carbon 1.05 content content Sintering Within 10° C. Sintering Within 10° C. window (1245-1255° C.) window (1275-1285° C.)

In the composition of the present invention, carbon (C) is a main element for forming carbides and improving the hardness and strength of the steel products. When the carbon content is less than 0.8 wt %, the liquid phase generation temperature will be greatly increased, and thus the sintering temperature is increased, which is not economic; and when the carbon content is higher than 1.40 wt %, the toughness of the sintered compacts will be lowered.

Silicon (Si) is capable of generating a thin oxide layer on atomized powders, which prevents the atomized powder from being further oxidized during cooling; however, excessively high silicon content will make the power oxide layer to be excessively thick, and thus blocking the sintering. Therefore, the optimal silicon content is lower than 1.0 wt %.

Manganese (Mn) is capable of improving the hardenability of the steel compacts; however, when the content is higher than 1.0 wt %, the oxygen content in the atomized powder will be greatly increased, and thus the powder cannot be sintered easily, and decarburization usually occurs during sintering. Therefore, the optimal manganese content is lower than 1.0 wt %.

Chromium (Cr) is capable of generating chromium carbide to improve the hardness of the steel compacts. Furthermore, when chromium is dissolved in the matrix, the corrosion resistance improves. The preferred chromium content is 15.0-18.0 wt %.

Molybdenum (Mo), vanadium (V), and tungsten (W) are capable of generating carbides upon tempering of the sintered steel compacts and thereby improving the hardness. The preferred content range of molybdenum, vanadium, and tungsten is 0.2-1.5 wt %. When the content is less than 0.2 wt %, the hardness cannot be improved. When the content is higher than 1.5 wt %, the effect for strengthening is gradually decreased and thus not being economic.

Titanium (Ti) is a strong carbide former. Titanium carbide is capable of effectively inhibiting the coarsening of the grains during sintering of the martensitic stainless steel powders, which resolves the problems of poor dimensional stability and poor mechanical properties of sintered steel compacts. The suitable amount of titanium added is 0.1-2.5 wt %, such that high sintered density and dimensional stability are obtained at a temperature range of 50° C. When the titanium content is less than 0.1 wt %, the effects of improving the dimensions and densities are not significant; and when the content is higher than 2.5 wt %, a titanium-containing pre-alloyed powder cannot be prepared easily, and the powder becomes expensive.

Next, the spirit of the present invention is described in detail below.

As for the sintering of the conventional martensitic stainless steels, when the temperature is raised above the liquid phase forming temperature, the generated liquid phase is capable of improving the diffusion and thereby enhancing the densification. But, unfortunately, the amount of liquid phase is very sensitive to the temperature. With too much liquid, distortion occurs. In contrast, with too little liquid, density is low. Moreover, the presence of the liquid phase will accelerate the diffusion of atoms and thus coarsening the grain. As a result, the total grain boundary area decreases and, accordingly, the thickness of the liquid phase increases. Therefore, the grain sliding caused by gravity becomes easier and induces deformation of the sintered compact.

In order to eliminate the phenomena above, in the present invention, titanium is added in the melt for atomization, such that a titanium carbide (TiC) or a titanium-containing composite carbide (Ti,V)C is formed in the atomized titanium-containing pre-alloyed powder. Such a titanium carbide will still exist in the base stably during the liquid phase sintering of the steel products and a fine grain structure will be obtained due to the inhibiting effect of grain boundary movement by titanium carbides. As the grain boundary increases, with the same amount of liquid phase, the thickness of the liquid phase among grains becomes thinner. As a result, the grain sliding becomes difficult and the work piece becomes intact without deformation. Accordingly, the sintering temperature range of the sinter can be broadened and high sintered density and good dimensional stability are obtained. Additionally, the strength, hardness, and toughness of the work piece will also be improved due to the fine grains.

In the present invention, a titanium powder, a titanium-containing pre-alloyed powder, or a titanium-containing carbide powder, such as TiC or (W,Ti)C, is pre-mixed in a base powder of martensitic stainless steels, molded with a molding process commonly used in the powder metallurgy industry such as dry compaction and powder injection molding, and then sintered. The method can alleviate the problems of poor dimensional stability and poor mechanical properties of sintered products.

The added titanium-containing carbide, such as TiC, and (W,Ti)C are stable during liquid phase sintering and has an excellent effect on inhibiting grain coarsening of the steel compacts. Since these sintered martensitic steel compacts are mostly used in high-wear environments, the particle size and content of the carbide in the matrix are very important factors in determining the wear resistance. The finer the particle size of the carbide is, the higher the ability of preventing the grain sliding and wear resistance. As for the selection of the particle size, the mean particle size of the titanium-containing carbide in the present invention is less than 5 μm.

In summary, all the ingredients in the steel powder compositions according to the embodiments of the present invention can effectively alleviate the dimensional control problems in sintering of martensitic stainless steel compacts and greatly improve the sintered properties.

The sintered body formed with the steel powder composition of the present invention also has a density close to those of cast or forged counterparts and has the advantages of improved dimensional stability and production yield. Compared with the sintering window of a conventional martensitic stainless steel powder compact of about 10° C., the sintering window of the present invention is expanded to 20° C.-50° C. 

1. A powder composition for martensitic stainless steels, comprising 0.80-1.40 weight percent (wt %) of carbon, greater than 0 wt % and less than 1.0 wt % of silicon, greater than 0 wt % and less than 1.0 wt % of manganese, 15.0-18.0 wt % of chromium, 0.7-2.50 wt % of titanium, and the rest of iron.
 2. The powder composition according to claim 1, further comprising 0.20-1.50 wt % of at least one of molybdenum, vanadium, and tungsten.
 3. The powder composition according to claim 2, wherein titanium in the steel powder composition is originated from a pre-alloyed powder.
 4. The powder composition according to claim 2, wherein titanium in the steel powder composition is originated from a titanium powder.
 5. The powder composition according to claim 2, wherein titanium in the steel powder composition is originated from a titanium-containing carbide powder.
 6. The powder composition according to claim 5, wherein the carbide powder has a mean particle size of less than 5 μm.
 7. The powder composition according to claim 2, wherein carbon in the steel powder composition is originated from graphite or carbon black powder.
 8. The powder composition according to claim 1, wherein titanium in the steel powder composition is originated from a pre-alloyed powder.
 9. The powder composition according to claim 1, wherein titanium in the steel powder composition is originated from a titanium powder.
 10. The powder composition according to claim 1, wherein titanium in the steel powder composition is originated from a titanium-containing carbide powder.
 11. The powder composition according to claim 10, wherein the carbide powder comprising titanium has an average particle size of less than 5 μm.
 12. The powder composition according to claim 1, wherein carbon in the steel powder composition is originated from graphite or carbon black powder.
 13. A sintered body, prepared from the powder composition according to claim 1 through a sintering process.
 14. The sintered body according to claim 13, further comprising 0.20-1.50 wt % of at least one of molybdenum, vanadium, and tungsten. 